1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic image. More particularly, the present invention relates to a developing device for use in electrophotographic image forming apparatus, such as copiers, facsimile machines and laser printers. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the developing device.
2. Discussion of the Background
In image forming apparatus using electrophotography, an electrostatic latent image corresponding to image data is formed on an image bearing member such as photoreceptors, and then the electrostatic latent image is developed with a developing device to form a visible image. In the developing device, a toner negatively charged by a charging device is transported to a developing area, and then the toner is moved onto an electrostatic image formed on a photoreceptor due to the electric field formed between the photoreceptor and a developing roller. Every time the toner is consumed in the developing device, a fresh toner is supplied to the developing device from a toner container.
In order to visualize an electrostatic latent image with a toner, single component developing methods, in which a thin toner layer formed on a developer bearing member comes close to or contacts an image bearing member, have been broadly used conventionally. In a developing device used for such single component developing methods, a toner layer forming member contacts a developer bearing member upon application of pressure thereto so that a thin toner layer is formed on the developer bearing member. On the other hand, the toner is charged by being frictionized with the toner layer forming member. However, such electrostatic developing methods have a problem in which a toner tends to be insufficiently charged, and thereby non-charged toner particles are transported to a developing area.
In attempting to solve such a problem, published Japanese Utility Model Application No. 63-138560 discloses a developing device including a corona charger. The corona charger supplies monopolar ions to a toner layer formed on the surface of a developer bearing member immediately above therefrom. However, when a corona wire is contaminated, nonuniform electrostatic discharging occurs resulting in unstable charging of the toner particles. In addition, the toner particles cannot be charged uniformly because the corona charger supplies ions to the toner particles only from one side. When toner particles, remaining on the developer bearing member after developing, are transported to a charging area, the difference in charged level between the residual toner particles and supplied toner particles is so large that the toner layer cannot be charged uniformly.
Published unexamined Japanese Patent-Application No. (hereinafter referred to as JP-A) 10-63096 discloses a developing device including a charge-decreasing device and a charging device for toner particles. The charge-decreasing device utilizes alternating-current corona occuring between a developer bearing member and a layer forming blade made of a conductive or semiconductive material. It is described in JP-A 10-63096 that combination of the charge-decreasing device and the charging device arranged on a downstream side from the charge-decreasing device relative to a rotating direction of the developer bearing member, can solve a problem such that toner particles cannot be uniformly charged when toner particles remaining on the developer bearing member after developing are repeatedly transported to a charging area. However, the developing device including the above-mentioned two charging devices is large in size, i.e., a compact developing device cannot be provided.